MAINT: minor cython cleanup in align/vector_fields.pyx

[grlee77]

These are minor changes to remove python interaction from within some of the loops present in vector_fields.pyx.

It is easiest to verify the reduction in Python interaction via observing the reduction in yellow lines in the annotated HTML files generated by cython -a vector_fields.pyx in this PR vs master.

The following four functions receive approximately 3 orders of magnitude speedup by avoiding calls to np.cos, np.arctan2 and/or np.sqrt:
create_circle, create_sphere, create_harmonic_fields_2d, create_harmonic_fields_3d

For the rest, there is negligable difference in performance. I have also looked at enabling parallel processing via prange in some of these functions and plan to submit that as a separate future PR. (In the multithreaded case, the change of _interpolate_vector_2d and _interpolate_vector_3d to use floating* out instead of floating[:] out makes a huge difference to some of the routines that were passing a view of a memoryview which was causing things to actually become slower if multithreaded)

[omarocegueda]

This looks good to me (sorry for the delay, I think this should have been merged immediately!)

[Garyfallidis]

Good job @grlee77!